1. Field of the Invention
The present invention relates to a motion detection device and method for use in motion adaptive signal processing of a video signal. The invention also relates to video signal processing devices and processing methods using the above motion detection device and method, and to a video display device using the processed video signal.
2. Description of the Related Art
Video signals are processed to enhance their displayed picture quality. The enhancement can be increased by detecting local image motion in the input video signal and carrying out motion adaptive signal processing. This is done by switching adaptively between processing suitable for still pictures and processing suitable for moving pictures, depending on whether the picture is locally still or moving. The two main types of motion adaptive signal processing carried out on television video signals are motion adaptive line interpolation and a type of three-dimensional (3D) noise reduction (NR) in which the strength of the noise reduction is controlled according to motion. Both of these types of signal processing are effective in improving picture quality.
Motion adaptive line interpolation is carried out when an interlaced television signal is enhanced by conversion to progressive scanning. In the interlaced-to-progressive scan conversion process (also called IP conversion), when the picture is detected to be still, inter-field interpolation is carried out by interleaving the lines of two temporally consecutive fields, thereby removing line flicker. When the picture is detected to be moving, intra-field interpolation is carried out by generating an interpolated signal from adjacent scanning lines in the same field.
In 3D noise reduction, each frame of the video signal is compared with the signal from one or more preceding frames and signal components that show no correlation across the plurality of frames are eliminated as noise. In infinite impulse response (IIR) noise reduction, the difference between the signal of the current frame and the noise-reduced signal of the preceding frame is multiplied by a coefficient and subtracted from the signal of the current frame. In finite impulse response (FIR) noise reduction, the noise is reduced by a filtering process in which the video signals of a plurality of temporally differing frames are multiplied by coefficients and added together. Both methods effectively remove noise components, which are uncorrelated on the time axis, from parts of the picture without motion. When motion is present, however, trails and ghosts appear and edges become blurred. To prevent these problems, a motion adaptive process is carried out by limiting the degree of noise reduction according to the amount of motion detected.
An effective way to obtain a motion signal indicating the amount of motion for motion adaptive signal processing purposes is to obtain a frame difference signal (frame-to-frame difference signal) by taking differences between the current input video signal (the current frame signal) and the signal of the immediately preceding frame (the preceding frame signal). When motion is detected only from frame-to-frame differences, however, detection errors occur. Part of a still picture may be incorrectly detected as moving, or part of a moving picture may be incorrectly detected as still. When motion adaptive signal processing is carried out, the accuracy of motion detection has a major impact on the quality of the displayed picture, so that motion detection processes that avoid such motion detection errors have been proposed.
In one of these proposed methods, motion is detected from the frame difference signal, after passing the frame difference signal through a vertical band-limiting low-pass filter (a vertical LPF), or using the spatial filter having a greater tap length vertically than horizontally, so as to carry out motion adaptive signal processing tailored to the spatial shape of the picture (see, for example, FIGS. 1 and 3 in Japanese Patent No. 2596166).
By applying a vertical filtering process to the frame difference signal or by detecting motion with a spatial filter having a longer vertical tap length as described above, the proposed method obtains a motion detection signal.
There is still the problem, however, that the frame-to-frame differences do not just reflect motion; they also include a noise component. Particularly at edges in the picture, the motion component and the noise component are intermixed, causing pixels that are actually still to be sometimes misrecognized as moving.
Moreover, when there is a slowly moving object or a moving object with a repetitive pattern of vertical or horizontal lines, calculation of differences between filtered frames can fail to detect motion, causing the picture to be incorrectly classified as still, because the calculated differences may be very small.
Thus in a picture that is even slightly unsteady or includes noise, frame differences due to the unsteadiness or noise affect motion detection by causing still pictures to be misrecognized as moving, and in a picture with slowly moving objects or moving objects with repetitive patterns, filtering of the frame difference signal can make it impossible to obtain values from which motion can be detected, so that the motion fails to be recognized.
These motion detection errors lead to degraded picture quality by causing flicker, blur, combing, and other problems. Combing is a problem in which the picture splits up in a comb-like pattern.
The present invention addresses the above problems with the object of providing a motion detection device and method that can perform highly accurate motion detection, without mistakenly detecting still parts as moving or moving parts as still, thereby reducing such forms of picture quality degradation as flicker, blur, and combing in the results of motion adaptive signal processing, and with the further objects of providing a video signal processing device, a video signal processing method, and a video display device using the above motion detection device and method.